The need for cooling electronic devices such as infrared detectors to cryogenic temperatures is often met by miniature refrigerators operating on the Stirling cycle principle. As is well known, these cryogenic refrigerators, or cryocoolers, use a motor driven compressor to impart a cyclical volume variation in a working volume filled with pressurized refrigeration gas. The pressurized refrigeration gas is fed from the working volume to one end of a sealed cylinder called a cold well. A piston-shaped heat exchanger or regenerator is positioned inside the cold well. The regenerator has openings in either end to allow the refrigeration gas to enter and exit.
The regenerator thus reciprocates in response to the volume variations in the working volume, and the refrigeration gas is forced to flow throw it in alternating directions. As the regenerator reciprocates, the end of the cold well which directly receives the refrigeration gas becomes much warmer then the ambient. In the other end of the cold well, called the expansion space or cold end, the gas becomes much colder than ambient. The electronic device to be cooled is thus mounted adjacent the expansion space, on the cold end of the cold well.
Because the cold well is sealed, the volume of the expansion space also varies as the regenerator reciprocates. It is known that the efficiency of the Stirling cryocooler is optimized by properly timing the movement of the regenerator. In particular, its movement should be such that the variations in the volume of the expansion space lead the variations in the volume of the compression space by approximately 90.. This insures that the working volume pressure and thus temperature are at a peak before the refrigeration gas enters the regenerator from the working volume.
The two most common configurations of Stirling cryocoolers are referred to as "split" and "integral". The split Stirling type has a compressor which is mechanically isolated from the regenerator. Cyclically varying pressurized gas is fed between the compressor and regenerator through a gas transfer line. In most split Stirling cryocoolers proper timing of regenerator movement is achieved by using precision friction seals.
In an integral Stirling cryocooler, the compressor, regenerator and cold well are assembled in a common housing. The typical arrangement uses an electric motor to drive the moving parts. A crankshaft, disposed in a crankcase, uses multiple cams to properly time compressor and regenerator movement, much as an internal combustion engine uses a crankshaft and cams to provide proper timing of the movement of its parts. As such, the typical integral cryocooler requires several bearings to support the cams and crankshafts. If connecting rods are used to couple the compressor and regenerator to the cams, additional bearings are required. One problem with this arrangement is that these bearings require a lubricant. Unfortunately, even the best of lubricants contain some minute amount of abrasives, and the moving parts eventually wear. Because efficient cryocooler operation requires maintaining extremely small, critical dimensional tolerances, even the minute contaminations carried in the lubricant cause unacceptable wear of the moving parts, which in turn severely shortens operating life.
One way to minimize this problem is to lower the pressure inside the crankcase. While this allows the bearings to be made smaller, thus decreasing the requirement for lubrication as well as the input power required to drive the moving parts, the lower pressure actually results in lower cooling efficiency. Thus, this is not a practical solution where it is also important to minimize power consumption.
Minimizing the size and weight of the bearings is also important where the entire cryocooler must be made as small and light weight as possible.
Another difficulty occurs with the crankcase. The normal arrangement is to pre-pressurize the crankcase through an access port. A lead o indium plug is then deformed into and around the port opening by a threaded set screw. The problem with this arrangement is that in order to obtain access to the crankcase at a later time, such as to repressurize, the plug must be cleaned out or scraped away to obtain access to the port.
Certain applications have traditionally dictated the use of split cryocoolers. For example, split cryocoolers are generally preferred in such applications as gimbal mounted infrared detectors, since only the regenerator and cold well need to be mounted on the gimbal, and the compressor can be remotely mounted. This reduces the weight of parts which must be mounted on the gimbal. Additionally, an integral cooler necessarily has a greater number of moving parts. Because moving parts transmit vibration to their environment, the need to mitigate vibration also sometimes dictates the use of split cryocoolers. However, spilt cryocoolers are normally expected to have a shorter operating life because their friction seals wear out more quickly.
In order to achieve maximum cooling efficiency, the device to be cooled must be mounted as close as possible to the expansion space. However, certain devices, such as mercury cadmium telluride detectors, are very sensitive to stress and strain. Thus, the minute vibrations caused at the regenerator cold end in response to the cyclical pressure variation have been found to adversely affect the operation of such detectors. The only solution to this problem previously has been to mount the detector farther away from the regenerator. However, this isolation between detector and regenerator adversely affects cooling efficiency.